Circuit and method for measuring current in a circuit

ABSTRACT

An electronic circuit for measuring current in a working circuit is provided with a first operational amplifier having a positive input, a negative input, and an output connected to at least one in-line amplifier to form a first current path. A second operational amplifier having a positive input, a negative input, and an output connected to a differential amplifier is provided within the circuit to form a second current path. An input terminal is connected to the negative inputs of each of the operational amplifiers to provide a connection between the electronic circuit and the working circuit. A variable conductance component is connected in series between the input terminal and the negative input of the second operational amplifier to control the flow of variable currents through the two current paths.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a process for measuring small fluxes aswell as a circuit for performing this process.

2. Discussion of the Related Art

In many measuring instruments such as, for example, mass spectrometers,vacuum gauges etc. the measurement values are produced on the basis ofcurrents which are generated by positively or negatively chargedparticles impinging on an electrode. The dynamic range of the ion orelectron currents which occur and have to be measured is very wide. Itfrequently spans from several hundred atto-Ampere to severalmicro-Ampere and thus covers over nine decades.

OBJECTS AND SUMMARY OF THE INVENTION

The present invention is based on the objective of recommending aprocess for the measurement of currents in this order of magnitude aswell as a suitable circuit for implementation of this method ofmeasurement, by which it is possible to produce the measured valuesquickly, accurately and without interferences.

The method of measurement of the present invention is characterised bythe features as herein described and claimed in the following methodclaims while the circuit of the present invention is characterised bythe features described below and claimed in the apparatus claims.

The special advantage of the method of measurement of the presentinvention is the fact that all current range outputs exhibit the sametransient response in parallel so that the measurement signals may beselected and read by a following multiplexer independently of themagnitude of the input currents. This makes logarithmic processingpossible by software, for example. The measurement values are producedvery quickly and this is supported by the advantage that the circuit canbe made very fast (with small time constants). Due to the short timeconstants, the averaging period can be selected within a wide rangethrough software by basing the average on many individual measurements.An offset can easily be accounted for by measuring all current rangeoutputs at a point of time when no input current is flowing, and bysubsequent subtraction of this value as the offset during the normalmeasurement process.

The circuit of the present invention is not complex and does not requiremuch space. The circuit does not require any relays and is thusinsensitive to interferences.

BRIEF DESCRIPTION OF THE DRAWING

Further objectives, advantages and details of the invention will becomeapparent from the following description taken in conjunction with thecircuits in the drawing, wherein:

FIG. 1 is a first schematic representation of a circuit according to thepresent invention; and

FIG. 2 is a second schematic circuit diagram of the present inventionshowing specific values for the components thereof.

DETAILED DESCRIPTION OF THE INVENTION

In the schematic diagram according to drawing FIG. 1 the input of thecircuit is marked 1. Input 1 is followed by an operational amplifier 2with its feedback resistor 3. The output 4 of the operational amplifier2 is connected via connection 5, into which a low pass filter 6 isinserted, to the multiplexer 7. Moreover, the output of low pass filter6 is also connected to the input of amplifier 9, the output 11 of whichis connected through connection 12 to the multiplexer 7. In addition,the output 11 of the amplifier 9 is connected to the input of amplifier13, the output 14 of which is connected though connection 15 to themultiplexer 7.

Moreover, a connection line 17 with two anti-parallel connected diodes18, 19 is connected to the signal input 1. This connection 17 leads tothe input of a second operational amplifier 21 with its feedbackresistor 22. The output 23 of this operational amplifier is connected toone of the inputs of a differential amplifier 25. The output 26 of thisdifferential amplifier is connected through connection 27 to themultiplexer 7 and in addition through connection line 28 to the input ofa further amplifier 29, the output 31 of which is connected throughconnection line 32 to the multiplexer.

Finally, the circuit comprises connection line 33 with the two seriesconnected diodes 34 and 35 as well as ground connected resistor 36. Aconnection line 37 is connected between diodes 34, 35 and the resistor36 which is connected to the second input of the differential amplifier25. Moreover, this connection is connected to the non-inverting input ofthe operational amplifier 21.

The output of multiplexer 7 is connected to the analogue/digitalconverter 38.

The presented circuit is capable of measuring input currents over a widedynamic range, i.e. when the current falls within the lower currentmeasurement range, a fairly small voltage is dropped across resistor 3,i.e. diodes 34, 35 are non-conducting. Therefore no current will flow inconnection line 33, so that no voltage drop can occur across resistor36. Thus the second operational amplifier 21 controls its minus input to0 V, so that also no voltage is applied to the diodes 18,19. Thus thesediodes are in the non-conducting state. The (small) input currenttherefore only flows into resistor 3 so that voltages can be measured atthe output 4 of operational amplifier 2 which are proportional to theinput current. The output 4 of the operational amplifier 2 thus formsone of the current range outputs which are connected to the multiplexer7.

The voltage signals caused by the very low input currents can also beamplified with the aid of amplifiers 9 and 13. The outputs 11 and 14 ofthese amplifiers are also connected to multiplexer 7 thereby formingfurther current range outputs. At the lowest input currents, i.e. whenthe output 14 is not overdriven, this output is used for themeasurements. At increasing input current levels, amplifier 13 willsupply an overdriven output so that one must change to the output 11 ofamplifier 9. As long as this amplifier 9 does not supply any overdrivensignals the signal supplied by output 11 is used as the measurementsignal. When the input current increases further, one will have tochange in the same way to output 4 of the operational amplifier 2.

Diodes 34 and 35 are designed in such a way, that they will start toconduct as soon as the measurement range in which signal range output 4of operational amplifier 2 supplies a proper measurement signal isexhausted. Then a voltage drop will occur across resistor 36 which dueto the controlling properties of operational amplifier 21 will alsoappear at its negative input. Thus the input current flowing into theinput 1 can split and is registered by both operational amplifiers 2 and21. The input current is split in such a way, that a constant componentflows into resistor 3, whereas the excess component in each case willflow into resistor 22 of the operational amplifier 21. The current rangeoutput 4 of the operational amplifier 2 will thus supply a constantvoltage signal to multiplexer 7 which lies within the linear range.Moreover, output 23 of the operational amplifier 21 supplies a signalwhich corresponds to the excess component, whereby this signal includesan error component from the forward voltage of the diodes 18 and 19.Since this voltage is also present at the input of the differentialamplifier 25 (connection line 37) the corrected voltage is available atthe output of the differential amplifier and this voltage is applieddirectly through connection line 26 or amplified once (amplifier 29) tomultiplexer 7.

At this point the circuit can be simplified, whereby the output ofamplifier 21 is directly connected to multiplexer 7. The correctionwhich is introduced in the presented example by differential amplifier25 must then be made software accessible by connecting connection line37 to the multiplexer 7 so that the difference may then be calculated bythe software. The current path for low currents may also have twooutputs and/or may be lead through a field-effect transistor. If a thirdcurrent path exists, then this current path may be linked to the second.The conductance of additional components which correspond to components18, 19 may preferably be derived from amplifier output signal 23. But itis also possible to connect the third current path directly to input 1and to derive the conductance of the components from amplifier output 4.

It is the task of the software which controls multiplexer 7, todetermine in each case which of the signal range outputs 4, 11, 14, 26,31 supplies a valid signal output which may be used as the measurementsignal. When starting from the most sensitive signal range output 14, itis possible to find the correct value based on overranging, for example.Only for the transition from the most insensitive current range output 4of the first group of amplifiers (2, 9, 13) to the most sensitive output31 of the second group of amplifiers (25, 29) the measurements must bemade already within the range which is related to output 31, because thevoltage of current range output 4 is maintained in the linear rangeafter the transition. The signal measured at output 4 must be added tothe signals at outputs 26 and 31, so that an output signal is obtainedwhich is proportional to the input current. This function is alsoimplemented by the software which controls the multiplexer 7.Analogue/digital converter 38 processes the signals for the processorwhich is connected to its output.

In those cases, where the input current changes so rapidly over manydecades that the storage time of diodes 18 and 19 makes itself felt,this undesirable influence can mostly be avoided by insertion of aswitch (a field-effect transistor, for example) into connection line 17which switches off at those currents where diodes 18 and 19 should benon-conducting.

Feedback components 3 and 22 of the operational amplifiers 2 and 21 arerepresented as resistors. Preferably, capacitors or non-linearcomponents may also be used. It is only important that the feedbackcomponents are selected in such a way, that an equal range expansionresults, i.e. that the stage of a higher current range is by a factor Vhigher compared to the last stage of the lower range. Amplificationfactor V depends on the required resolution in the transition range.Smaller amplification factors also result in a lower resolution at thebeginning of each range. If five outputs exist, for example, then thefollowing results: ##EQU1##

Shown in drawing FIG. 2 is a dimensioned example according to drawingFIG. 1 for the measurement of currents produced by positively chargedparticles. The following applies with respect to the diodes 34, 35 whichdetermine the transition of the measurement signal based on a singlesignal, to the measurement signal based on summed signals: when ADconverter 38 has been designed for a maximum voltage of 5 V theoperating voltage of the diodes 34 and 35 should be somewhat less, 4.7V, for example, i.e. the input current in the transitional range wouldamount to: ##EQU2##

When certain voltage levels are exceeded, the multiplexer changes to thenext input. This is done as follows: The software reads the values fromthe most sensitive input. When the value which is read-in corresponds tothe full scale value of the AD converter, the multiplexer must switch tothe next less sensitive range and a value will have to be read in again.This is repeated until the output of the AD converter is not longer atits full scale.

While this invention has been described in detail with reference tocertain preferred embodiments, it should be appreciated that the presentinvention is not limited to those precise embodiments. Rather, in viewof the present disclosure which describes the best mode for practicingthe invention, many modifications and variations would presentthemselves to those of skill in the art without departing from the scopeand spirit of this invention, as defined in the following claims.

What is claimed is:
 1. An electronic circuit for measuring current in aworking circuit, said electronic circuit comprising:a first operationalamplifier having a positive input, a negative input, and an outputconnected to at least one in-line amplifier forming a first currentpath; a second operational amplifier having a positive input, a negativeinput, and an output connected to a differential amplifier forming asecond current path; an input terminal connected to the negative inputsof each of the operational amplifiers, said input terminal beingconnectable to the working circuit to receive an input signal therefrom;and variable conductance means connected in series between said inputterminal and said negative input of said second operational amplifier.2. The electronic circuit according to claim 1 wherein high currentinput signals are distributed between said first and second currentpaths so that a constant component of the input signal is directed intothe first path and an excess component thereof is directed in to thesecond path, said excess component being amplified by said secondoperational amplifier and said differential amplifier to produce a validcurrent signal at an output thereof, the desired measured signal beingthe sum of said constant component and said valid current signal.
 3. Theelectronic circuit according to claim 2 including a first and a secondin-line amplifier connected in series with the output of said firstoperational amplifier.
 4. The electronic circuit according to claim 3further including a low pass filter connected in series between theoutput of said first operational amplifier and said first in-lineamplifier connected.
 5. The electronic circuit according to claim 4further including a third in-line amplifier connected in series with anoutput of said differential amplifier.
 6. The electronic circuitaccording to claim 5 further including a multiplexer wherein the outputof said differential amplifier and a respective output of each of saidfirst, second, and third in-line amplifiers, and of said low pass filteris connected into said multiplexer.
 7. The electronic circuit accordingto claim 6 further including an analogue/digital converter connected tosaid multiplexer.
 8. The electronic circuit according to claim 1 whereinsaid variable conductance means includes two diodes, connected inanti-parallel, that change their conductance by a voltage change at saidpositive input of said second operational amplifier, said differentialamplifier correcting an error voltage at the output of said secondoperational amplifier caused by a forward voltage on the diodes.
 9. Theelectronic circuit according to claim 8 further including a pair ofseries connected diodes connected in series between the output of eachof said first and second operational amplifiers, and a resistorconnected in series between the output of said second operationalamplifier and ground, said pair of series connected diodes and saidresistor causing said voltage change at said positive input of saidsecond operational amplifier.
 10. A method for measuring current in aworking circuit, said method comprising the steps of:providing a firstcurrent path through a first operational amplifier having a positiveinput, a negative input, and an output connected to a pair of seriesconnected in-line amplifiers; providing a second current path through asecond operational amplifier having a positive input, a negative input,and an output connected to a differential amplifier being seriesconnected to a single in-line amplifier; connecting an input terminal tothe negative inputs of said first and second operational amplifiers;directing low level currents from the working circuit into the inputterminal; blocking the low level currents from the second current pathso that the entire current flow is directed into the first current path;selecting a valid current signal at an output of the series connectedpair of in-line amplifiers; and measuring the value of the selectedvalid current signal.
 11. The method according to claim 10 wherein saiddirecting step includes directing a constant component of higher currentlevels from the working circuit into the first current path anddirecting a remaining excess component of the current into the secondcurrent path.
 12. The method according to claim 11 including the furtherstep of amplifying the excess component of current directed through thesecond current path.
 13. The method according to claim 12 wherein saidselecting step includes selecting a valid current signal at an output ofthe single in-line amplifier of the second current path.
 14. The methodaccording to claim 13 including the further step of adding the constantcomponent of current in the first path to the valid current signalselected from the second path, and said measuring step includes thecombined current value from both current paths.
 15. The method accordingto claim 14 wherein said into the second current path through the secondoperational amplifier, the differential amplifier, and the singlein-line amplifier.
 16. The method according to claim 10 wherein saidblocking step includes blocking current flow into the first currentpath.
 17. The method according to claim 16 wherein said measuring stepincludes measuring a valid current signal at an output of the singlein-line amplifier in the first current path.
 18. The method according toclaim 11 including the further step of:directing current signals fromboth the first current path and the second current path into amultiplexer; and connecting the multiplexer to an analogue/digitalconverter to provide digital signals compatible with a software routinefor further processing and control of the multiplexer.
 19. The methodaccording to claim 18 wherein said blocking step is performed by avariable conductance component connected in series between the negativeinput terminals of the first and second operational amplifiers, thevalue of the variable conductance component changing as a function of anoutput signal at the output of the first operational amplifier to blocklow current levels and conduct higher current levels.
 20. The methodaccording to claim 18 wherein all values provided at the current pathoutputs are measurable and recordable in the case of a missing inputsignal and wherein during said measuring step these values aresubtracted as offset values.
 21. The method according to claim 18wherein an error voltage at the output of the second operationalamplifier is applied to the multiplexer and corrected by the softwareroutine.